Stopping bird-window collisions at home

Birds cannot see clear glass, and glass reflects sky and trees (American Bird Conservancy). Cornell's All About Birds says collisions are most frequent at dawn and dusk during spring and fall migration. The fix that every source on this page agrees on is simple: mark the outside of the glass with a pattern dense enough that a bird reads it as a barrier. The sources differ on how dense, so this page lays their numbers side by side.

How many birds die at windows?

Audubon says three more victims typically go unseen for every one found, and the Bird-window Collision Working Group (BCWG), whose collaborators include Muhlenberg College's Acopian Center for Ornithology, says up to 50% of collisions leave no evidence at all.

Among Loss and colleagues' species most vulnerable at residences were Purple Finch, Ruby-throated Hummingbird and Ovenbird. ABC's 2022 list of seven species most likely to collide with windows, a rough order by overall collision frequency, starts with White-throated Sparrow, Ovenbird and Common Yellowthroat, and includes the Dark-eyed Junco at number four.

The 2x4 rule, the 2x2 rule and what each source says

The "2 x 4 Rule" appears in the BCWG's BirdSafe Homes brochure without a definition. The U.S. Fish and Wildlife Service describes its "2-inch by 4-inch spacing rules": horizontal stripes at least ¼ inch wide no more than 2 inches apart, or vertical stripes at least ¼ inch wide no more than 4 inches apart, with closer spacing such as 2 by 2 inches needed for smaller birds and for dots.

ABC adds that larger marks work better and white tends to perform best.

Step by step: treating your windows

1. Find the problem windows. ABC says a small number of windows usually cause most collisions: ones that have already been hit, large windows and glass doors that reflect habitat or sky or that you can see sky or greenery through, and windows across from feeders, birdbaths or fruiting plants; Cornell adds paired windows at right angles.
2. Look from a bird's side. Check each window from a bird's point of view (Cornell). If indoor plants are visible from outside at window level, treat that glass or move the plants (USFWS).
3. Choose an outside treatment. ABC, Cornell and USFWS name tape or dots, tempera or acrylic paint, closely spaced decals, hanging cords, full insect screens and one-way films. Outside is "always most effective" (USFWS); ABC says inside treatments can be invisible when outside reflections are strong.
4. Space the marks. Use the table above. Two inches or less in every direction meets every source.
5. Size the marks. At least ¼ inch for dots and ⅛ to ¼ inch for stripes (ABC, USFWS, FLAP Canada), and visible from 10 feet (ABC).
6. Test an inside option if you must. For a window you cannot reach, ABC suggests putting sticky notes inside and checking from outside every two to three hours from early morning; if they stay clearly visible, birds may see them too.
7. Turn off lights you don't need, especially during spring and fall migration (USFWS, FeederWatch).
8. Keep watching. Fixing the worst windows may stop nearly all collisions; watch for new strikes elsewhere (ABC).

Replacing windows?

ABC warns that collisions often rise after replacement (more reflective glass, partial or between-pane screens, bigger undivided panes). It advises full exterior screens, even on windows that don't open, plus a treatment on the new glass or bird-friendly glass.

What does not work well

• A hawk silhouette. Cornell says hawk silhouettes do little; FLAP Canada says a single silhouette is not an effective deterrent.
• One or two stickers on a large window (Cornell). Decals only work when closely spaced.
• UV-only decals for every bird. ABC notes that birds that cannot see ultraviolet, such as Mourning Doves, may not be protected by UV-reflective products.
• Markings on the inside of reflective glass (ABC, USFWS).

Feeders and windows: how far apart?

Sources split into two camps: feeders very close to the glass, or treat the glass and forget distance.

The close-feeder advice traces to experiments such as Klem et al. (2004) in Pennsylvania. With feeders 1 m (about 3.3 ft) from test windows, none of the collisions were fatal; the fatal share was 2% at 2 m, 59% at 5 m and 69% at 10 m (about 33 ft). If you use a window feeder, see the notes in bird feeder types; if a hawk is hunting at your feeders, see hawks at bird feeders.

If a bird hits your window

1. Check it gently. Approach from behind (Cornell, Audubon). Audubon suggests gently moving its legs; if they don't move, the bird is dead.
2. Contain it. Cover it gently with both hands, without squeezing, and place it in an unwaxed paper bag or a small box lined with tissue or paper towel (Cornell). Avoid containers with holes, loops or strings that can tangle legs (Audubon).
3. Keep it dark, quiet and warm, away from children and pets. Don't handle it, and don't give it food or water (Cornell, Audubon).
4. Call a wildlife rehabilitator. Cornell and Audubon both recommend it; Audubon calls it the best option. If none is available, Audubon suggests leaving the bird undisturbed for about an hour, then releasing it in a yard or park if it is moving around. If a rehabilitator tells you to release it, Cornell says to do so toward vegetation away from buildings and to call again if it flies poorly.

A bird that keeps attacking its own reflection is a different problem: Cornell says it is most common in spring and rarely fatal. See the Northern Cardinal page for that.

Window checklist

• List the windows that reflect trees or sky, face feeders, or have already been hit
• Pick an exterior treatment: tape, dots, paint, cords, screens or film
• Gaps no more than 2 in (4 in only for vertical lines or cords, if you follow the 2x4 rule)
• Dots at least ¼ in, stripes at least ⅛–¼ in, visible from 10 ft
• Feeders within 3 ft of the glass (or treat the glass anyway)
• Indoor plants out of view from outside
• Unneeded lights off during migration
• A paper bag or small box and a rehabilitator's number ready

More feeder placement tips: what to feed backyard birds.

Sources

• Why Birds Hit Windows—and How You Can Help Prevent It — All About Birds, Cornell Lab of Ornithology
• Solutions for Homes — American Bird Conservancy
• Glass Collisions — American Bird Conservancy
• These Seven Bird Species are Most Likely to Collide with Windows — American Bird Conservancy (2022)
• Reducing Bird Collisions with Buildings and Building Glass: Best Practices — U.S. Fish and Wildlife Service (2016, updated 2021, PDF)
• Federal Agencies Treating Glass to Reduce Bird Collisions — U.S. Fish and Wildlife Service (2023)
• Loss, Will, Loss and Marra (2014), Bird–building collisions in the United States: estimates of annual mortality and species vulnerability, The Condor 116:8–23 (PDF hosted by Audubon)
• Klem et al. (2004), Effects of window angling, feeder placement and scavengers on avian mortality at plate glass, The Wilson Bulletin 116:69–73 — summary by Conservation Evidence
• BirdSafe Homes brochure — Bird-window Collision Working Group, Acopian Center for Ornithology, Muhlenberg College (PDF)
• Safe Feeding Environment — Project FeederWatch
• How Can I Feed Birds Safely? — FLAP Canada
• Reducing collisions with glass — Audubon (2020)
• You Found a Bird That Crashed Into a Window. Now What? — Audubon (Carlyn Kranking, 2021)
• File:Dotted glass bird collision deterrent wwf poland.jpg — Wikimedia Commons (photo source)